The present invention relates to an optical device for the correction of aberrations affecting an image. In particular, a device according to the invention makes it possible to correct the distortion due to a spherical concave mirror that is inclined with respect to the direction at which this mirror is observed.
The invention can be applied to a helmet viewfinder for the pilot of a fighter aircraft or helicopter or for the operator of a training simulator.
A helmet viewfinder is an image-presenting device that is integrated into a helmet. The viewfinder enables the wearer of the helmet, for example a pilot of an aircraft in flight, to observe visual information simultaneously with the view of the landscape or of the pilot""s cabin, which he perceives most usually through a protective visor. The presentation of appropriate information, for example in the form of symbols, provides piloting and navigation assistance. Thus, for armed vehicles, the presentation of a reticule provides assistance in the aiming of a weapon. The information may also consist of an image of the landscape acquired by sensors other than the eye of the helmet wearer such as infrared image sensors or light intensifiers to complement or replace direct viewing.
Inside the helmet, an image generator comprises an imager whose screen, for example a cathode-ray tube screen or a liquid crystal screen, enables an image to be displayed.
The image is most usually conveyed by a relaying optic up to a combiner which presents the conveyed image in a state where it is superimposed on the view of the landscape.
In order that the pilot may simultaneously observe the landscape viewed directly at infinity and the image from the imager, the latter image is also focused at infinity by a collimation optic.
When the combiner is formed by a simple semi-reflective plane plate, the collimation of the image can be achieved by an optic placed between the imager and the combiner; a prior art embodiment such as this has the major drawback of requiring a collimation optic that requires far too much space in relation to the restricted field of view that is obtained.
To reduce the space requirement, a combiner with optical power has been proposed; a combiner such as this provides its user with both the collimation of the image and the superimposition of the collimated image with the view of the landscape.
The prior art has a very extensive variety of devices comprising a combiner with optical power. Of more particular interest are image-presenting systems that comprise a spherical concave mirror to collimate the image.
A concave spherical mirror achieves an average quality collimation of an image placed at a particular point in space located on the axis of the mirror and at a distance from this mirror equal to half its radius of curvature. By placing an imager at this point, the eye located on the axis of the mirror receives rays coming from the imager after they are reflected on the spherical mirror. These rays are parallel and lead to the perception, by the eye, of a collimated image. If, furthermore, the mirror is semi-reflective, it enables the same eye to observe the landscape by transparency. However, in a device such as this, the imager would have to lie on the axis of the semi-transparent spherical mirror and it would conceal the user""s field of view.
To clear the user""s view, the spherical mirror is inclined with respect to the normal to his/her face and the user""s eye is no longer on the axis of the mirror. This arrangement has the drawback of resulting in a collimated image that is affected by optical aberrations, especially off-centering aberrations, which need to be corrected, at least partially.
The inclination of the spherical concave mirror afflicts the collimated image with distortion, known as off-centering distortion of the second kind, characterized by a convergence of the verticals and an apparent curvature of the horizontals.
The prior art teaches us in order to correct the distortion of the image provided by the optical assembly to introduce an inverse distortion at the imager level by electronic correction; this is easily achieved when the imager comprises a cathode-ray tube but this solution is not suited to an imager, such as for example a light intensifier, which does not exhibit the necessary adjustments of the image. One could also attempt to correct the distortion by inserting into the optical path between the imager and the spherical mirror another inclined spherical mirror introducing distortion inverse to the first; however, one would end up with an unusable optical system owing to its bulkiness.
In a patent filed under U.S. Pat. No. 9,709,893 on Aug. 1st, 1997 by the present Applicant, an aspherical mirror with an adapted shape enables a correction of the off-centering distortion of the second kind.
The particular surface of the proposed aspherical mirror enables a modification of the light rays in order to rectify the effects of the spherical concave mirror on the horizontals and verticals of the image observed and thus ensure a correction of the distortion. This correction is achieved by the introduction through the aspherical mirror of an off-centering distortion of the second kind to compensate for the distortion of the same type due to the spherical concave collimating mirror used off-axis. The aspherical mirror has the effect of making the verticals parallel and the horizontals rectilinear in the collimated image. The image is rectified and orthoscopic but the overall shape of the mirror causes a local amplification of the aberrations, and especially of astigmatism. The correction of the distortion enabled by this invention is limited by a deterioration of the resolution of the image.
The problem is to construct a device for the presentation of images comprising an off-axis spherical collimating mirror presenting a collimated image that is satisfactory for the user, namely an image that is devoid of troublesome aberrations and has a wide field of view greater than or equal to 40 degrees. This entails obtaining a collimated image that has both high resolution and high correction of the distortion.
The spherical collimating mirror being observed at an oblique angle with respect to its axis, it introduces an off-centering distortion of the second kind characterized by an absence of symmetry of revolution. This distortion is especially dangerous for a user piloting a vehicle, since the perception of perspective is degraded. The difficulty consists in finding a means of correcting the distortion which does not degrade the quality of the image and is such that the entire optical device has restricted mass and restricted bulk.
This is why the invention proposes an optical device for a helmet viewfinder presenting a collimated image to a user, comprising an imager and an off-axis spherical concave mirror, characterized in that it comprises optical means for correcting the distortion of the image presented to the user which is due to the off-axis spherical concave mirror, said means comprising a diffractive field mirror.
The diffractive mirror comprises a reflection hologram. According to the invention, the correction of the distortion is effected when the diffractive mirror is placed in a vicinity of an intermediate image of the optical device: this is a diffractive field mirror. Its diffractive effect in proximity to the intermediate image makes it possible to displace the points of the image nonuniformly.
In the vicinity, the correction carried out by the diffractive mirror does not degrade the resolution of the image. The extent of the vicinity is limited by the resolution, which is imposed by the remainder of the device. The diffractive mirror is preferably situated at the limit of the vicinity fixed by the resolution. While being within the vicinity of the intermediate image, the diffractive mirror is placed a maximum distance from the intermediate image beyond which it degrades the resolution of the image presented to the user.
The diffractive mirror can be, for example, situated in the vicinity of the first intermediate image, that is to say that which is closest to the eye of the user of the device. However, it is preferably placed in the vicinity of the second intermediate image; this preferred arrangement allows the construction of a lighter and less bulky device in which the hologram is better protected.
The hologram in the invention is, for example, digital numerical with discrete variations, plane numerical with continuous profile, it may also be recorded in a photosensitive material.
The substrate in which the hologram is fashioned may be plane, but it is preferably not plane; the curvatures of the surface taking on board some of the correction, the hologram affording a residual correction.
The transparent substrate of the volume hologram in a photosensitive layer preferably exhibits a variable optical index or a variable thickness.
The device also comprises one or more optical power groups or optical relay groups placed in the path of the rays between the imager and the spherical mirror, upstream and/or downstream of the diffractive field mirror.
These optical groups also afford precorrection of the astigmatism necessarily introduced into the collimated image owing to the fact that the spherical mirror is observed at an inclined angle with respect to the ray which defines the optical axis of this mirror. This astigmatism can be corrected for example by a spherical convergent lens and a cylindrical lens, in an optical relay group situated between the imager and the diffractive mirror. It can also be corrected by a diffractive lens placed in a power group between the diffractive mirror and the spherical mirror.
The invention makes it possible to preserve an image of good resolution whilst affording a thorough correction of the distortion due to the spherical and inclined collimating mirror. The invention has the advantage of correcting the distortion of the image presented to the eye of the user for a wide instrument pupil, for example at least 15 millimeters in diameter, and for a wide field of typically greater than 40 degrees. The instrument pupil is the zone in space in which the user of an instrument must place the pupil of his eye in order to use it.
This correction is especially beneficial when a distortion cannot possibly or cannot easily be imposed at the level of the imager. Indeed, in such a case an electronic correction of the prior art is not suitable.
The first pupillary image of the device is inclined with respect to the optical axis, the diffractive mirror according to the invention gives a second, upright, pupillary image standing on the optical axis.
The invention can be incorporated into a helmet viewfinder exhibiting a wide instrument pupil and a wide field.